Electronic components, such as integrated circuits (ICs), are typically assembled into component packages by physically and electrically coupling them to a substrate made of organic or ceramic material. One or more component packages, such as IC packages, can be physically and electrically coupled to a printed circuit board (PCB) to form an “electronic assembly”. The “electronic assembly” can be part of an “electronic system”. An “electronic system” is broadly defined herein as any product comprising an “electronic assembly”. Examples of electronic systems include computers (e.g., desktop, laptop, hand-held, server, etc.), wireless communications devices (e.g., cellular phones, cordless phones, pagers, etc.), computer-related peripherals (e.g., printers, scanners, monitors, etc.), entertainment devices (e.g., televisions, radios, stereos, tape and compact disc players, video cassette recorders, MP3 (Motion Picture Experts Group, Audio Layer 3) players, etc.), and the like.
In the field of electronic systems there is an incessant competitive pressure among manufacturers to drive the performance of their equipment up while driving down production costs. This is particularly true regarding the packaging of ICs, where each new generation of IC packaging must provide increased performance, while maintaining good yield and reliability.
A high performance IC typically has a relatively large number of input/output (I/O), power, and ground terminals (also called “bumps” herein). An IC package substrate has a number of metal layers selectively patterned to provide metal interconnect lines (also called “traces” herein), and a relatively large number of terminals (also called “pads” herein) to which the terminals of an IC can be suitably connected, for example, using solder.
To enhance the reliability of the solder joints connecting the IC bumps and the IC substrate pads, underfill encapsulant is used to mechanically and physically reinforce them. In a known method of underfill encapsulation, a low viscosity epoxy material is dispensed along one or two edges of an assembled package, allowing the underfill to be drawn into the gaps between the IC and the substrate by capillary action, and the underfill is subsequently cured using heat. However, this method requires separate operations to perform solder reflow, dispense the underfill, clean up any excess underfill, facilitate underfill capillary flow, and perform underfill cure, thus increasing the overall manufacturing costs. Also, with the die bump pitch and bump height decreasing and terminal count increasing, it becomes successively more difficult to obtain adequate underfill dispersion through capillary forces alone.
High performance ICs generate sufficient heat and may also be exposed to sufficient ambient heat to cause reliability problems in the form of cracked bump-to-pad connections, if the coefficient of thermal expansion (CTE) of the underfill material is significantly higher than that of silicon and/or the IC substrate material, e.g. FR-4. It is known to add certain materials, e.g. silica particles, to the underfill to lower its CTE, as well as to stiffen the underfill. However, adding particles increases the underfill viscosity, making it more difficult to apply through capillary forces.
It is known to use a no-flow underfill that is applied to the IC mounting area without using capillary forces, the underfill being subsequently cured concurrently with solder reflow, as described, for example, in U.S. Pat. No. 6,180,696. However, if sufficient particles are added to the underfill to lower its CTE, the particles tend to cause a significant interconnection yield problem, because they get interposed between the IC bumps and the substrate pads and prevent good solder joints.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a significant need in the art for methods for applying underfill to component packages, such as IC packages, that minimize yield and reliability problems.